The present invention relates to fused fiber optic coupler and wavelength division multiplexer (WDM) devices. More specifically, but without limitation thereto, the present invention relates to a method for modifying the coupling ratio and wavelength dependence of these devices.
Fiber optic couplers and wavelength division multiplexers are generally manufactured by placing two optical fibers in parallel contact, applying heat to fuse them together over an appropriate length, thereby forming a coupling region, and applying tension on the ends of the optical fibers to impart an elongated taper to the coupling region.
FIG. 1 shows the power transfer through the coupling region of a typical fiber optic coupler as a function of elongation of the coupling region. After reaching a maximum, the coupling ratio oscillates in a sinusoidal manner. It is believed that the sinusoidal envelope associated with the greater elongation is due to polarization birefringence in the coupling region.
Wavelength division multiplexers, such as described in U.S. Pat. No. 5,121,453 filed on Jun. 9, 1992 and incorporated herein by reference thereto, may be made by halting the elongation process after a predetermined number of oscillations of the coupling ratio, or power transfer cycles, have occurred. In general, the more power transfer cycles (i.e., the greater the elongation or taper), the narrower the wavelength channel spacing will be as determined by the distance between power transfer minima and maxima relative to wavelength. The tapering process is halted at a point preferably as close as possible to a maximum coupling ratio (preferably 100%) at the operating wavelength, with a minimum coupling ratio (preferably 0%) at the wavelength of the desired channel spacing. This is done to minimize crosstalk between channels.
Precise halting of the tapering process is hindered by several factors, however. For example, after many power transfer cycles, changes in the coupling ratio occur rapidly with changes in optical fiber length, so that a small change in optical fiber length results in a relatively large change in the coupling ratio. In addition, once the tapering process is halted, the fused structure is usually packaged to prevent changes in the coupling ratio from occurring due to mechanical stress. Typically this is done by epoxying or soldering the optical fibers to a substrate. The process of attaching the optical fibers to the substrate may change the tension applied to the optical fibers, however, causing a change in length and a corresponding change in the coupling ratio or shift in the operating wavelength. The packaged device thus often fails to have the desired coupling ratio at the specified operating wavelength, and may even be rendered useless for the intended application, resulting in decreased yields and higher unit costs.